1. Field of the Invention
The invention generally relates to laser scanner systems based upon semiconductor light sources for reading indicia of different light reflectivity such as bar code symbols, and more particularly, to scanner systems in which adjustment of the intensity or spatial coverage of the beam is utilized to effect the detection and reading of symbols.
2. Description of the Prior Art
Various types of laser scanning systems are known in the prior art that read bar code symbols. Bar code symbols are applied to a label or surface of an article to form digital representations of information that are used to identify the article or other characteristics thereof. The bar code symbol itself is a coded pattern of indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths. A laser scanner reads the symbol and decodes it into the digital representation representing the encoded information.
Various types of scanner systems are exemplified by U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,593,186; 4,496,831; 4,409,470; 4,460,120; 4,607,156 and 4,673,803--all of such patents being owned by the assignee of the instant invention and being incorporated by reference herein. Such scanners have generally been designed to read bar code symbols, particularly of the Universal Product Code (UPC) type, and operate at a certain working or reading distance from the symbol at a hand-held or stationary position.
Typically, a scanner includes a light source such as a laser or semiconductor device that generates a light beam which is optically modified to form a beam spot of a certain size. It is preferred that the beam spot size be relatively small, generally the same as or less than the minimum width between regions of different light reflectivity, i.e. the bars and spaces of the symbol. However, in practice it is not actually necessary that the spot size be so limited. The relative size of the bars and spaces is determined by the type of coding used to represent characters, as is the actual size of the bars and spaces. The number of characters per inch represented by the bar code symbol is referred to as the density of the symbol.
The beam is directed by optical components along a light path toward a target that includes a bar code symbol on the surface. A scanning component may either sweep the beam spot across the symbol and trace a scan line across and past the symbol, or scan the field of view of the photodetector, or do both. A scanner also includes a sensor or photodetector. The photodetector has a field of view extending across and slightly past the symbol and functions to detect light of variable intensity reflected off the symbol. The photodetector generates electrical signals representative of the sequence of bars and spaces in the symbol. The electrical signals are then decoded into data descriptive of the symbol.
The use of semiconductor devices as the light source in scanner system are especially desirable because of their small size, low cost and low power requirements. However, there are some drawbacks to the use of semiconductor devices in some applications.
First, some semiconductor lasers, especially those emitting light in the visible portion of the spectrum, have a relatively limited lifetime. The reflective facets which define the lasing cavity are relatively thin layers which degrade in reflectively over time. As a result, the effective output power of the laser also declines, which limits the operational effectiveness of the scanner.
As we will note later, since such degradation is proportional to the current applied to the diode. Of course, if the current applied is lowered, the device will have a longer lifetime.
In addition to gradual degradation, other degradation mechanisms are (1) catastrophic degradation, and (2) dark-line defect formation.
In catastrophic degradation, the laser mirror under high-power operation is permanently damaged by pits or grooves forming on the mirror.
The dark-line defect is a network of dislocations that can form during laser operation. Once started, it can grow expansively in a few hours, causing the threshold current density to increase.
Another disadvantage of the use of semiconductor lasers is that the rise in temperature associated with constant-on or high duty cycle operation will further reduce lifetime.
Still another disadvantage of semiconductor lasers presently available commercially is their relatively low visibility, compared, for example, to the visibility of the beam from a HeNe gas laser. Such low visibility is due to their high wavelength, about 680 nanometers.
Prior to the present invention there has not been a satisfactory way to overcome the disadvantages of using a visible semiconductor laser in a scanning system for reading bar code symbols.